1. Field of the Invention
The present invention relates to a method of restoring degraded image, an apparatus for restoring degraded image, and a computer program product.
2. Description of the Related Art
Conventionally, as an image restoration technique for restoring a degraded image, there are a method of using a generalized inverse filter and the Richardson-Lucy method (also referred to as the Landweber method) (see M. R. Banham and A. K. Katsaggelos, “Digital Image Restoration”, IEEE Signal Processing Magazine, pp. 24-41, 1997). In both methods, a desired restored image is determined so that a square error between an image degraded by a degradation process and an input image becomes the smallest, assuming that the degradation process is known.
The generalized inverse filter calculates an inverse matrix as a result of solving the condition that square error becomes the smallest. Because the calculation can be difficult as it is, discrete Fourier transform is performed to execute calculation in a frequency space, and the calculation is returned to a real space again. This is because the inverse matrix operation becomes division in the frequency space.
According to the Richardson-Lucy method, a restored image having the smallest square error is obtained by gradual convergence from an initial solution to a real solution by an iteration method. For example, in JP-A 2002-300459 (KOKAI), a memory-saving configuration is shown such that a local convolution operation is performed based on the Richardson-Lucy method, thereby making it unnecessary to perform a large-scale matrix operation.
The generalized inverse filter can obtain a restored image by one processing; however, the large-scale matrix operation is required, thereby causing a problem that a calculation amount is large. Further, because the Richardson-Lucy method is an iterative solution, iteration of from several times to several tens of times is required to obtain a desired restoration result. The same applies to an image restoring apparatus described in JP-A 2002-300459 (KOKAI), which is based on the Richardson-Lucy method. As the number of iterations increases, the calculation amount also increases, thereby making it difficult to realize the generalized inverse filter by an apparatus or the like.